An injection molding machine which melts feedstock composed of thermoplastic resin and injects it into a mold is constituted to include a screw which is accommodated in the inside of an injection cylinder. Then, a screw with only one blade-shaped flight spirally provided on the circumferential surface of a shaft, which is rotationally driven, has been widely used as the screw. In such a screw, solid thermoplastic resin feedstock supplied to the inside of the injection cylinder is kneaded while being melted and plasticized by receiving a shear force from the screw which rotates, and then is injected into the mold.
Incidentally, as means for improving the kneadability of thermoplastic resin, a so-called double flight screw provided with two flights has been used in recent years. The double flight screw is constituted by being provided with one sub-flight having a lower height and a larger pitch than a main flight between adjacent main flights (refer to, for example, PTL 1 and PTL 2).
Here, a double flight screw disclosed in PTL 1 is formed such that the thickness of the gap between a leading end of a sub-flight and the inner wall surface of an injection cylinder has a constant magnitude. On the other hand, a double flight screw disclosed in PTL 2 is formed such that a difference in the outer diameter between a main flight and a sub-flight gradually decreases toward the leading end side of the sub-flight, that is, such that the thickness of the gap between the leading end of the sub-flight and the inner wall surface of the injection cylinder gradually decreases toward the leading end side of the screw.